Settling of fluorescent screens



Patented Dec. 15, 1953 UNITED No win A at on May '27, .1 52;

serial No. 290,345

C aims mi itr, application Gr at B ain June .1951

-2 claims. (01. I i -3 3:5)

1 The present invention relates to -the settling of fluorescent screensand more particularly tothe depositing of such screens .on glass orglass like surfaces such-as the face of aca'th'o'de rayf tilbe.

In the manufacture of fluorescent screens, the

application of the fluorescent -fnaterial involves two main factors.First, the Zinateirial must be applied uniformly to the surface orbacking jand :second, ,a binder must be employed to cause it to adhereetO the glass. one method iujsed hereto- ;fore-has been to apply abinder to the glass backing and then pply the fluores ent material s afine powder which is allowed tofseittle'jon to the binder. ln iahothermethod a suspension jof the powdered fluorescent ,inate'r'ial in 'abinder is sprayed onto the glass backing. I In accordance with a stillfurther metrica the fluorescent material is heldi'n suspension in aliquid and allowed to settle onto the glass backing, binder materialhavin be n a d d t, a Su t b stag and the surplus liquid being finallyjdecan ed. The present invention is concerned With fthijs la'ih termethod. 7 r

The main problem in this settling process of forming a fluorescentscreen is that of combining quality .in the finished product with speedof manufacture, since the settling and subsequent adhesion of theiiuorescentm'aterial from :t-hasuspension is liable to take some time,foraexample, one or two hoursor more which in quantity zpiO-ductionis-anundesirablylongtiine.

For the purpose of improving Ithe quality and uniformity of screensproduced by this method as well as to decrease the tota1 settling time,a process has been developed in which a suspension of the phosphorparticles in an alkali silicate solution is settled through a diluteaqueous solution of an electrolyte such as barium nitrate or bariumacetate as more fully described and claimed in copending applicationSerial No. 157,185 of Friedman, filed April 20, 1950. While this methodgives a screen layer having good adhesion, the settling time necessarybefore the surplus liquid can be poured or decanted off is stillsubstantial. It is believed that the effectiveness of this process isdue to the presence of the soluble barium salt. The barium salt hastensthe gelation of the potassium or sodium silicate by entering intochemical reaction with the silicate to form an insoluble alkaline earthsilicate precipitate which acts as a binder for the phosphor.

The present invention is based on the discovery that furtherimprovements can be obtained in this settling process forming afluorescent screen by using a second electrolyte in addition to the 2 shn barium 's'alt. second e e trolyte is a sparingly soluble .icalciuiiisal specifically. 'et' bbra w h al o ac with h a kal silicate to give aninsoluble silicate. P eteri an the -ar na metab t i mix d with the:suspensisn' f the fluorescent material n the lka'li silicate solution,and this suspension is their ppuree en top "of a cushioning layer of aSolution bf-b'a'riu'm nitrate. I

EXP rim t hav nd ated tha n Prd r to o ain the esired resm s, it isneces ary that both or the clear "as i. the fsolubl barium al and tannin.be em l yed I br exam le, a ifi qrestent inanuract'ured by fi a ma h ls w r e m with 1 cushioning layer "of pure water and thereafter adding amixture .of calcium metaborat'e, ftlie phosphor and poi'ljets 'u'msilicate Wil have good adhesion [after a re) minutes set, me but whenthe ex ess 'settl'ingmedium is poured Toff, the edges of thefluorescent, layer tend to slip dowh'into theusja'blejfacefareajof thetu e.

t u face or the 'partines or the fluorescent mittena1 by reaction of thecalcium metaborate with the alkali silicate greatly reduces thisconsolidation time, thus allowing the decantation or pouring off step tobe performed much sooner.

The alkali silicate solution employed in the practice of the presentinvention is preferably a potassium silicate solution obtained bydiluting a commercially available solution with an equal orapproximately equal amount of Water. A commercially available solutionhas, for example, the following specifications: molecular ratio lKzO 3.9S102, percentage K207.8%, and specific gravity=1.25. By diluting such asolution with an equal part of water, there is obtained a dilutepotassium silicate solution having a specific gravity of about 1.1. v 50The cushioning layer of barium salt solution preferably has a bariumnitrate or acetate concentration of about 0.5 gram per liter ofsolution. While it is highly desirable to work very close to this bariumsalt concentration, satisfactory re sults can be obtained when theconcentration 3 ranges from about 0.4 to 0.6 gram barium salt per litersolution.

As the total quantity of the silicate suspension added to the cushioninglayer and forming the remainder of the total settling medium isrelatively small as compared with the quantity of the cushioning layer,the barium salt concentration of the entire settling medium may also beconsidered as being approximately within the limits of 0.4 to 0.6 gramper liter.

The concentration of the calcium metaborate is also critical. Ordinarilythis electrolyte will be employed in amounts ranging from about 0.04 to0.06 gram per liter of total settling medium or solution. Calciummetaborate is only sparingly soluble in water, and its solubility fallswith the reduction in the number of molecules of water ofcrystallization. Best results are obtained by using a calcium metaboratehaving the approximate formula Ca(BOz)2H2O which can be obtained byheating a fully hydrated calcium borate containing 6 molecules of waterin air to about 350 C. for 3 to 4 hours. It has been found that too higha. borate solubility gives a fiocculent precipitate of calcium silicate,which, on settling, gives undesirable inert spots on the screen layer,whereas too low a borate solubility gives poor adhesion and consequentslipping cf the phosphor layer, particularly at the edges.

The present process is, of course, not limited in its application tosettling a specific phosphor and may be used, for example, with the zincsulphite, zinc cadmium sulphide phosphors, zinc silicate phosphors,magnesium silicate phosphors, zinc beryllium silicate phosphors, andzinc oxide phosphors.

The following is a specific example illustrating the manner in which theprocess of the present invention may be carried into effect.

Into a 12" round bulb or tube was poured 4,000 cc. of a .05% bariumnitrate solution. After this solution had come to rest, there was addedthrough a distributing rose, 400 cc. of a suspension of phosphor andfinely-divided calcium metaborate in a potassium silicate solutionhaving a specific gravity of 1.1. The quantity of phosphor was 2.5 gramsand of calcium metaborate 0.25 gram. The calcium metaborate contained 1molecule of Water of crystallization. After 15 minutes the bulb wastilted on a tilting table and 5 the excess settling medium poured off.The remaining phosphor layer was of good quality and 4 possessedexcellent adhesion with no indication of slip at the edges. The settlingtime required to obtain these results was quite short considering thelow concentrations of electrolytes employed.

Experiments have shown that these results appear to be specific to thecalcium metaborate. For example, the substitution of calcium hydroxidefor the calcium borate does not result in any noticeable improvement ofthe wet adhesion of the screen layer. Likewise, the substitution ofsodium borate for the calcium borate showed no improvement in the wetadhesion of the fluorescent layer.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The process of settling a phosphor coating onto the face of a cathoderay tube which comprises covering the face of said tube with acushioning layer comprising a dilute solution of a soluble barium saltselected from the group consisting of barium nitrate and barium acetate,said solution containing from about 0.4 to 0.6 gram of said barium saltper liter of solution and thereafter adding to said cushioning layer asuspension of a phosphor in a potassium silicate solution containing asmall amount of calcium metaborate, the amount of calcium metaborate insaid silicate solution being such that the concentration thereof in thetotal settling medium including the silicate solution and the cushioninglayer is from about 0.04 to 0.06 gram per liter.

2. In the process of forming a phosphor coating on a glass surface bysettling the phosphor onto the surface from an aqueous settling mediumcomprising potassium silicate and a suspension of a powdered phosphor,the steps of including in said medium a quantity of barium nitrate in aconcentration of about 0.5 gram per liter and calcium metaborate in anamount ranging from about 0.04 to 0.06 gram per liter of settlingmedium.

ARTHUR JOHN HALSTEAD DARLASTON.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,108,683 Leverenz Feb. 15, 1938 2,412,654 Sadowsky Dec. 17,1946 2,451,590 Tidik et al. Oct. 19, 1948 2,487,097 Byler Nov. 8, 1949

1. THE PROCESS OF SETTLING A PHOSPHOR COATING ONTO THE FACE OF A CATHODERAY TUBE WHICH COMPRISES COVERING THE FACE OF SAID TUBE WITH ACUSHIONING LAYER COMPRISING A DILUTE SOLUTION OF A SOLUBLE BARIUM SALTSELECTED FROM THE GROUP CONSISTING OF BARIUM NITRATE AND BARIUM ACETATE,SAID SOLUTION CONTAINING FROM ABOUT 0.4 TO 0.6 GRAM OF SAID BARIUM SALTPER LITER OF SOLUTION AND THEREAFTER ADDING TO SAID CUSHIONING LAYER ASUSPENSION OF A PHOSPHOR IN A POTASSIUM SILICATE SOLUTION CONTAINING ASMALL AMOUNT OF CALCIUM METABORATE, THE AMOUNT OF CALCIUM METABORATE INSAID SILICATE SOLUTION BEING SUCH THAT THE CONCENTRATION THEREOF IN THETOTAL SETTLING MEDIUM INCLUDING THE SILICATE SOLUTION AND THE CUSHIONINGLAYER IS FROM ABOUT 0.04 TO 0.06 GRAM PER LITER.